This invention relates to a hydraulic control arrangement for a roof support unit of a longwall mineral mining installation having a conveyor and a plurality of roof support units positioned side-by-side along the goaf side of the conveyor, and in particular to control means for controlling the advance of shield extensions of the roof shields of the roof support units.
The shield extensions of such an installation are advanced towards the face being won by hydraulic rams associated with the corresponding roof shields. The shield extensions support the roof in the mineral mining working in the critical region adjacent to the face. It is important, therefore, to advance the shield extensions as soon as possible after the conveyor has been advanced following a cutting run of the plough (or other winning machine) along the face side of the conveyor. Known systems for advancing the shield extensions incorporate either manual control means or automatic control means.
In one known automatic control system, special sensors are provided on the shield extensions, these sensors serving to sense the position of the face, and to control the advance of the shield extensions in dependence upon their distance from the face. Such a system is relatively expensive. Moreover, this type of system is not very reliable, since the face does not constitute a well-defined limiting surface.
The aim of the invention is to provide a hydraulic control arrangement for a roof support unit which does not suffer from these disadvantages.